The basic unit (pixel) of an electrochromic display is an electrochemical cell that may change colour when an electric current goes through the cell. Examples of electrochromic displays are ion-intercalation displays with solid or porous films of e.g. WO3 and TiO2, displays with electrochromic polymer films, metal (e.g. silver) electrodeposition displays, displays with electrochromic action in the electrolyte (containing e.g. viologen molecules), and displays with viologens or other electrochromic agents anchored to the huge inner surface of a porous nanostructured metal-oxide film (e.g. a TiO2 film). These and other types of electrochromic systems are described in more detail in e.g. P. M. S. Monk et al., “Electrochromism: Fundamentals and Applications”, VCH, Weinheim 1995; SE0002834; U.S. Pat. No. 6,067,184; WO9835267; M. O. M. Edwards, Electrochimica Acta 46 (2001) 2187; and in references therein.
In directly addressed displays each pixel is connected by a separate electric conduction line (at least to one of the pixel electrodes) to an external drive voltage source, facilitating simultaneous individual electrical control of all pixels in the display. When the number of pixels in a display is very large, typically more than 100, it is either physically impossible or impractical to connect one separate line to each pixel. To overcome this problem the pixels are commonly arranged in a matrix structure in which they are addressed by time-multiplexing techniques via row and column lines from the matrix edges. Such displays and the methods of addressing them are denoted matrix displays and matrix addressing, respectively. In active-matrix displays all pixels are equipped with an electronic circuit with at least one field-effect transistor, diode, metal-insulator-metal diode, or another kind of non-linear electronic component. Matrix displays without pixel circuitry are called passive-matrix displays. The pixel circuitry in active-matrix displays improves the addressing properties of the pixels, however, the manufacturing of active-matrix displays is more complicated and expensive. The present invention relates to electrochromic passive-matrix displays.
A typical arrangement for an electrochromic passive-matrix display with m×n pixels is schematically described as follows. The electrochromic material, the electrolyte, films, and other components of the electrochromic cell are sandwiched between two electrodes of conductive glass. The front electrode (facing the viewer) and the back electrode are divided in m and n, respectively, electrically isolated conduction lines. The widths of the isolations are narrow compared to the widths of the conduction lines. The lines of the front and back electrodes are perpendicular with respect to each other. A pixel is defined by the crossing between a front line and a back line. There are many variants of this basic arrangement. For instance: The front and back lines must not necessarily be perpendicular; the lines may be curved and have irregular shapes. The front and back lines may be situated on the same substrate. The electrode materials may be others than conductive glass. In addition to the isolations in the electrodes, the pixels may be further isolated from the surrounding pixels by seals, patterned structures in films, etc.
Several addressing schemes for passive-matrix displays are known. Time-varying electric signals are applied on the conduction lines of both electrodes. Typically the lines of one of the electrodes (e.g. the front electrode) are used as select lines to select a group of pixels for receiving colour information from the data lines, i.e. the conduction lines of the other electrode. In the most straightforward addressing scheme (SCHEME 1) this is carried out by applying one of two voltages (USELECT or UUNSELECT) on the select lines and one of two voltages (UDARK or ULIGHT) on the data lines. Pixels along one of the select lines are selected by applying USELECT on this particular select line and UUNSELECT on all other select lines. The voltages on the data lines to the selected pixels are UDARK for dark pixels and ULIGHT for light pixels. After a certain time tline the selected line is changed and new information (UDARK, ULIGHT) is applied on the data lines according to the colour states of the pixels along the new selected line. This procedure is repeated until all m select lines in the matrix have been selected once. The process of scanning through all select lines is called a frame. A frame is immediately followed by a new frame as long as an image is shown. The same voltage sequences of UDARK and ULIGHT are applied on the data lines in all frames as long as the image is not changed. When the image is changed new voltage sequences are applied on the data lines. To avoid flickering the frame time tFRAME (m×tline) is often shorter than 1/60 s.
SCHEME 1 is an example of a single-line addressing scheme. It works well for e.g. certain liquid-crystal displays and is described in “Liquid Crystal Displays” by Ernst Lueder (ISBN 0471 49029 6). The basic idea is that dark pixels are darkened only when they are selected and that the darkness to a large extent remains during one frame until next time the pixel is selected. The light state is here taken as the “normal state” for unselected pixels, but the opposite situation is also possible (is taken for granted in the discussion throughout the text). For SCHEME 1 to work properly the display type should preferably show (1) a sufficiently sharp threshold in the colour-voltage relation as well as (2) a short-circuit memory during the non-selected part of a frame. The first condition is to assure that only the applied pixel voltage Upixel=USELECT−UDARK corresponds to a dark state, whereas the three other possible voltage differences (USELECT−ULIGHT, UUNSELECT−UDARK, UUNSELECT−ULIGHT) correspond to light states. The second condition is to assure that the obtained dark state for a selected pixel that is darkened is to large extents unaffected by the applied pixel voltages (UUNSELECT−UDARK, UUNSELECT−ULIGHT) during the time of the frame it is not selected.
An alternative single-line addressing scheme (SCHEME 2) uses three voltages (USELECT-DARK, UUNSELECT, USELECT-LIGHT) on the select lines and two voltages (as in SCHEME 1) on the data lines. The ‘normal’ colour state in this scheme is ‘grey’ (something between dark and light). Briefly the scheme is based on superframes consisting of two different kinds of frames: dark frames and light frames. The applied voltages on unselected lines are UUNSELECT (as in SCHEME 1) in both, whereas USELECT-DARK and USELECT-LIGHT are applied on the selected lines during dark and light frames, respectively. UDARK and ULIGHT are applied on the data lines to dark and light pixels, respectively, in both kinds of frames. The intention is, however, that dark and light pixels are mainly darkened and lightened, respectively, in the dark and light frames. In other respects SCHEME 2 works like SCHEME 1.
Another kind of addressing than single-line addressing is multiple-line addressing. In multiple-line addressing schemes several select lines are selected simultaneously. Multiple-line addressing schemes for liquid-crystal displays are described in “Liquid Crystal Displays” by Ernst Lueder (ISBN 0471 49029 6). The electric signals on the select lines in those schemes are voltage waveforms that, in a mathematical respect, are orthogonal to each other.
The following definitions are used throughout the text: Pixels along the same select line are in the same row. Pixels along the same data line are in the same column. Pixels with different visual colour impressions are in different “greyscale states”. For instance, greyscale states can be black, dark grey, light grey, and white as well as blue, green, and red. We will also use the term greyscale state for pixels that in an image switch are changing from one greyscale state to another one. Pixels that change from yellow to blue are in a yellow→blue state whereas pixels in which a red colour is kept are in a red state (or red→red state). We will also talk about ‘desired’ greyscale states. We will then mean that the desired greyscale states of, e.g., yellow→blue and red states are blue and red, respectively.
A known problem (see EP0000616) when SCHEME 1 is applied on electrochromic passive-matrix displays with two greyscale states (light, dark) is related to the greyscale uniformity of the pixels in the matrix. The greyscale state of a dark pixel is dependent on the number of dark pixels in its column. The more dark pixels in a column, the darker appearance of the dark pixels in that column. Consider, as an illustrating example, a matrix with 7 rows and 5 columns. The voltage across an unselected pixel is (neglecting the resistances in the select and data lines) either UUNSELECT−UDARK or UUNSELECT−ULIGHT. A dark pixel in a column with 5 dark pixels will in the unselected periods of a frame experience 4 periods of UUNSELECT−UDARK and 2 periods of UUNSELECT−ULIGHT. As a comparison, a dark pixel in a column with only 1 dark pixel will experience UUNSELECT−ULIGHT in all 6 unselected periods. From this it is obvious that the voltage driving of a pixel in the unselected periods of a frame is dependent on the number of dark pixels in the pixel's column. As a result of this, matrix displays that lack “short-circuit memory” will have non-uniform greyscales. (By short-circuit memory we mean that the pixel colour is not affected by the applied pixel voltages in the unselected periods of a frame. Note that the greyscales for a matrix display with perfect short-circuit memory would be uniform with SCHEME 1.)
A driving method SCHEME 4 for overcoming the said greyscale uniformity problem is disclosed in EP0000616. In SCHEME 4 the line addressing periods in SCHEME 1 are divided in two equally long sub-periods A and B. Voltage waveforms with different voltages in A and B are used for USELECT, UDARK and ULIGHT, whereas UUNSELECT always is 0 V in both A and B. UDARK and ULIGHT are, respectively, +U′ and −U′ in A and −U′ and +U′ in B. In this way the time-average of the pixel voltage in a line-addressing period, <Upixel>, becomes always 0 V for all unselected pixels in the matrix. For example, <Upixel>=<UUNSELECT−UDARK>=((0−U′)+(0+U′))/2 V=0 V for an unselected dark pixel. As <Upixel> is the same for all unselected pixels, the greyscale states of dark pixels will be independent of the number of dark pixels in a column, provided that the bleaching is a proper function of only <Upixel>. A drawback of the method is that pixels only are darkened in one of the sub-periods A and B. For a matrix with 7 rows, this means that dark pixels are actively darkened only 1/14 of a frame, as compared to 1/7 of a frame with SCHEME 1. The maximum contrast between dark and light pixels will therefore be lower with SCHEME 4 than with SCHEME 1. This is the price one pays for the better contrast uniformity with SCHEME 4.
One problem with prior art (SCHEME 4) is that the contrast between dark and light pixels is low. Another problem is that no methods for more than two greyscale states are provided. Another problem is that no methods for fast image switching are provided.